Fluid fossil fuels, such as oil and natural gas, are becoming scarce as these fuels are consumed by a world whose population is continually growing. Also, the supply of oil to industrialized countries has been and could be again interrupted for political reasons. For these reasons, much attention is being directed towards pyrolyzing carbonaceous material to liquid and gaseous hydrocarbons.
A problem encountered in the pyrolysis of many carbonaceous materials, such as coal, is recovery of hydrocarbons from the pyrolytic vapors. These pyrolytic vapors typically contain entrained finely divided solids and hydrocarbons having a wide boiling range from components as light as methane up to heavy viscous fractions which are commonly known as "tars." In slow pyrolysis such as experienced with coke ovens, primary tar fragments produced recombine to form coke and crack to form gas, coke, and a lower yield of secondary tar. By contrast, when coal is heated to its final temperature rapidly and the vapors are immediately condensed, a higher yield of complex primary tar results. Whereas this technique offers significant advantage in ultimate yield, the condensation and recovery of the primary tar is more difficult.
The tar produced by pyrolysis of coal is a complex mixture of compounds with a wide range of molecular weight. The recovery, purification, and subsequent processing of this material to fuel oil or synthetic crude oil is difficult because of its unfavorable physical and chemical properties. These include high viscosity, surface tension, low hydrogen to carbon ratio, and polymerization and cracking tendencies. In processes which give a high yield of liquids such as rapid pyrolysis, these factors are pronounced. For example, when pyrolyzing bituminous coal, the tar in the pyrolytic vapor is so viscous it cannot be poured at room temperature.
Attempts to lower the viscosity of the tar by operating a recovery process at higher temperatures results in degradation of the tar. Also, the tar contains the particulate matter entrained by the pyrolytic vapors. These solid particles are difficult to remove from the tars because of the high viscosity of the tar, and the small particle size, often less than 10 microns, of the entrained solids.
The presence of appreciable quantities of entrained solids in the tar derived from solid carbonaceous materials is highly undesirable. Equipment employed in the handling and subsequent treatment of the tar may become fouled due to the presence of the fine solid materials with impractical maintenance and replacement costs thereby resulting. In addition, at temperatures characteristic of pyrolysis treatments of solid carbonaceous materials, it is possible that fine spent material, when present in relatively large proportions, may act as a catalyst and effect polymerization of the tar.
Moreover, liquid hydrocarbons derived from carbonaceous solids containing significant quantities of solid materials in finely divided form usually are unsatisfactory for commercial utilization. The presence of suspended solids gives rise to fouling of equipment employed in refining, pumping, storing and like handling operations with resultant increase in the overall maintenance and replacement costs of such items.
Solid carbonaceous pyrolysis residue in the tar is characterized by having sensible heat value and, since it also contains fixed carbon, combustion heat value. In some cases the amount of fines materials entrained in the tar may attain a value as high as above 10 percent of the total carbonaceous solids charged to the pyrolysis zone. In order to maintain the efficiency and economy of such methods utilized to recover the desired hydrocarbon values from the solid carbonaceous material at practical levels, it generally is imperative that the heat value of the entrained residue be recovered.
Attempts to recover primary tar produced by rapid heating such as in fluidized or entrained beds by contacting pyrolytic vapors with water to condense, coalesce, and scrub hydrocarbons from the vapor have been ineffective. This is because a portion of the high molecular weight liquid droplets do not readily coalesce but instead remain entrained as fine droplets in the presence of noncondensible gases present in pyrolysis processes, and particularly in rapid pyrolysis processes which employ recycled gas in addition to the gaseous products. In addition, the liquid product collected is not fluid at temperatures practical for a water scrub system.
To overcome these problems, some processes have employed absorbers, with recycling of product tar to directly cool, condense, and scrub tar from pyrolytic vapor, such as the COED process as reported to the Office of Coal research in R & D Report #56 and Interim Report #1. Typically, this type of operation is performed stagewise, where the highest boiling fraction and char fines are collected in the first stage. The stream from the first stage is then recycled to the pyrolysis reactor because of its high solids content and because complete separation of solids from this stream is uneconomical. In the pyrolysis reactor, the heavy liquid polyaromatic hydrocarbons in this stream yield mostly char and gas, which are generally less valuable than liquid hydrocarbons. Thus the COED recovery method results in lowering the liquid yield of the pyrolysis process.
In addition, to minimize the amounts of liquids lost due to recycle to the pyrolysis reactor, the first stage of the COED process is operated at high temperature. This results in condensation of a very viscous tar fraction which, during process upsets resulting in loss of temperature control in the first stage and during shutdown, can solidify and plug the processing equipment. Also, solids can escape the first stage and contaminate the product oil. Thus separation of solids from the product tar is still necessary especially when fixed bed hydrotreatment is used because such char can plug the bed. This solids separation step is difficult because of the high viscosity of the full range tar and small density difference between the tar and suspended solids and the small particle size of the solids.
Thus, there is a need for a process for recovery of hydrocarbons from pyrolytic vapors which permits separation of hydrocarbon fractions covering a wide range of boiling points, easy handling of the tars recovered, removal of entrained solids from the hydrocarbon fractions, and easy hydrogenation of the tar to upgrade the value of the tar and to prevent its self-polymerization.